This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of a relatively low viscosity hydrocarbon followed by an aqueous surfactant slug.
In the recovery of oil from oil-bearing reservoirs, it is usually possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques has been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus it has been proposed to add surfactants to the injected water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions in order to reduce the oil-water interfacial tension are commonly referred to as low tension waterflooding techniques. To data one of the more promising low tension waterflooding techniques involves the injection of aqueous solutions of petroleum sulfonates within a designated equivalent weight range and under controlled conditions of salinity. For example, in a paper by W. R. Foster entitled "A low-Tension Waterflooding Process", Journal of Petroleum Technoloqy, Vol. 25, Feb. 1973, pp. 205-210, there is disclosed a procedure which involves the sequential injection of a protective slug, a surfactant slug, and a mobility control slug. The protective slug is an aqueous solution of sodium chloride which is injected in order to displace the reservoir water ahead of the subsequently injected surfactant slug. This slug is substantially free of divalent ions which would tend to precipitate the surfactant slug. It, as well as the surfactant slug, may contain inorganic sacrificial agents such as sodium carbonate and/or sodium tripolyphosphate which function to improve the water wettability of the reservoir rock surfaces and reduce adsorption of the surfactant.
The surfactant slug comprises an aqueous solution of petroleum sulfonates having an average molecular weight within the range of 350-500 in concentrations ranging from about 1.0-3.0 weight percent. The surfactant slug contains sodium chloride in a concentration, typically about 1.0 to 2.0 weight percent, which will promote the desired low interfacial tension between the injected water and the reservoir oil. The subsequently injected thickened water slug contains a viscosifier such as a water-soluble biopolymer in a graded concentration in order to provide an initial viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. This mobility control slug has a lower sodium chloride concentration than the surfactant slug. This somewhat lower salinity functions to increase the desorption of the previously adsorbed surfactant to move the surfactant through the reservoir by a chromatographic-desorption process.
Various modifications of, or alternatives to, surfactant waterflooding involve the injection of a surfactant and a hydrocarbon slug or the injection of surfactants in both a hydrocarbonaceous solution and an aqueous solution. For example, U.S. Pat. No. 3,468,377 to Dunlap et al. discloses the injection of an aqueous solution of petroleum sulfonates having a median molecular weight within the range of about 375 to about 430. The aqueous surfactant solution may be preceded by a hydrocarbonaceous solution of surfactants in a volume of about one-tenth of to about equal to the volume of the aqueous solution with the total volume of the hydrocarbonaceous and aqueous solution being from about 0.01 to about 0.2 pore volume. U.S. Pat. No. 3,491,834 to Ahearn et al . discloses the injection of a nonpolar (hydrocarbon) slug containing a preferentially oil-soluble sulfonate surfactant followed by a polar (aqueous) slug containing a somewhat lower molecular weight sulfonate which is preferentially water-soluble. The size of the nonpolar slug is said to be between 0.5 percent and 20 percent of the reservoir pore volume and preferably between 2 percent and 10 percent. The polar slug varies from 0.5 percent to 100 percent of the pore volume, preferably from 25 percent to 75 percent, and may contain a thickening agent.
U.S. Pat. No. 3,865,187 to Carlin et al. discloses an oil recovery process which involves an emulsification mechanism resulting from the injection of a hydrocarbon solvent containing a mono-unsaturated secondary alcohol followed by an aqueous solution containing a sulfate salt of a fatty alcohol. Each of the respective slugs varies in size from about 5 percent to about 50 percent reservoir pore volume with the alcohol present in a concentration within the range of 0.1-10 percent by weight and the alcohol sulfate being present in amounts from about 0.1 to about 2.0 percent by weight. The aqueous slug may be followed by water containing a thickening agent in an amount from about 0.01 to 0.5 weight percent. Another process disclosed in U.S. Pat. No. 2,669,306 to Teter et al. involves the injection of a liquefied normally gaseous hydrocarbon such as propane, followed by the injection of drive water. The patentees disclose that recovery of hydrocarbons may be improved by the addition of surface-active agents selected so as to avoid emulsification difficulties.